unimaginable tonight. In this crushing heat fighting fire and carrying 40 pounds of gear. How do they endure it? Jindsy janice is with them. Reporter: Diane, we are standing in what was someone's house. This right here the garage door. Firefighters are calling this time of day the bye witching hours, when the winds and heat kick up creating perfect conditions for the fire to start running again. The fire is exploding in size, covered in bone dry shrubs, rocket fuel for the flames. We are the national priority right now. Reporter:3,000 firefighters and support crews are struggling to outsmart it. Those in the front lines up against the toughest of conditions. You're sweaty, you've been hydrating prior but you can't seem to drink enough water. Temperatures of 100 plus and you have the heat from the fire. Reporter: Firefighters on the front line carry food, water and firefightering tools. Total weight can reach up to 45 pounds. This year's record breaking heat making the challenge even greater. In california, just a one degree increase from the average summer temperature can mean 35% more lantd is burned. The job is nowhere near done with only 15% of this blaze contained. Firefighters told us they're carrying at least a gallon of water up the mountain with them. They say they simply can't drink enough of it. Diane? We're always in awe of what those firefighters do.

This transcript has been automatically generated and may not be 100% accurate.